You're suggesting that Adobe hasn't 'improved things', when in fact, they've been releasing new versions faster than ever before. 10.2 was just recently released, with support for 'StageVideo', allowing HD movies to be rendered via the graphics pipeline, giving native performance because it uses your hardware.
They improved their cursor support, hooking into the OS to provide 'native' cursors, rather than just drawing them in Flash.
IE rendering speed was increased by 33% due to new hooks into their 'native' graphics pipeline, they improved their OSX integration, however they didn't release performance numbers.
Their sub-pixel rendering has been updated, removing optimizations for CRTs, giving crisper, cleaner lines on IPS displays.
10.3 beta was just announced, which will provide echo cancellation for audio input, improved analytics for video-based sites, integration into browser privacy settings (no more super-cookie), and a 'local' settings panel for all Plugin settings.
Also on their labs site is their 'incubator', which provides a full and true 3D rendering API, though the release date for this has not been announced.
So, while you can speculate on what their incentives might be, you certainly cannot fault Adobe for not updating and improving their plugin.